The present invention relates to methods of manufacturing paperboard. More particularly, the invention relates to a paperboard manufacturing method that enables greater stiffness and strength for multi-ply paperboard of the same fiber furnish basis weight as compared to prior art methods.
Paper is manufactured by an essentially continuous production process wherein a dilute aqueous slurry of cellulosic fiber flows into the wet end of a paper machine and a consolidated dried web of indefinite length emerges continuously from the paper machine dry end. The wet end of the paper machine comprises one or more headboxes, a drainage section and a press section. The dry end of a modem paper machine comprises a multiplicity of steam heated, rotating shell cylinders distributed along a serpentine web traveling route under a heat confining hood structure. Although there are numerous design variations for each of these paper machine sections, the commercially most important of the variants is the fourdrinier machine wherein the headbox discharges a wide jet of the slurry onto a moving screen of extremely fine mesh.
The screen is constructed and driven as an endless belt carried over a plurality of support rolls or foils. A pressure differential across the screen from the side in contact with the slurry to the opposite side draws water from the slurry through the screen while that section of the screen travels along a table portion of the screen route circuit. As slurry dilution water is extracted, the fibrous constituency of the slurry accumulates on the screen surface as a wet but substantially consolidated mat. Upon arrival at the end of the screen circuit table length, the mat has accumulated sufficient mass and tensile strength to carry a short physical gap between the screen and the first press roll. This first press roll carries the mat into a first press nip wherein the major volume of water remaining in the mat is removed by roll nip squeezing. One or more additional press nips may follow.
From the press section, the mat continuum, now generally characterized as a web, enters the dryer section of the paper machine to have the remaining water removed thermodynamically.
Contemporary food and small article packaging relies heavily upon a roughly 0.009 in. caliper or greater thickness of paper broadly characterized as paperboard. Two of the more desirable qualities sought for paperboard packaging are stiffness and surface smoothness. High stiffness relates to the speed at which the paperboard may be controllably transferred through a converting machine. Surface smoothness relates to the quality of sales promotional graphics that may be transferred to the paperboard surface by traditional printing processes.
In recent years, fourdrinier machines have been developed to make paperboard having multiple, independent layers or plies of papermaking stock laid together or in closely spaced sequence along a single forming section of the fourdrinier screen circuit. What is referred to herein as layers or plies is to be distinguished from a laminated composite of independently formed solid sheet having a sharply defined interface between juxtaposed sheet surfaces. In the case of multi-ply fourdrinier-formed paper or paperboard, such as the present invention, each of the "layers" or "plies" could more accurately be described as a "zone" that transitions substantially seamlessly into the adjacent zone. The interface is not a plane but a transition zone of proportionately significant thickness wherein the fiber of adjacent zones are commingled.
Generally speaking, the most important fibers for the manufacture of paper are obtained from softwood and hardwood tree species. However, fibers obtained from straw or bagasse have been utilized in certain cases. Both chemical and mechanical defiberizing processes, well known to the prior art, are used to separate papermaking fiber from the composition of natural growth. Papermaking fiber obtained by chemical defiberizing processes and methods is generally called chemical pulp whereas papermaking fiber derived from mechanical defiberizing methods may be called groundwood pulp or mechanical pulp. There also are combined defiberizing processes such as semichemical, thermochemical or thermomechanical. Either of the tree species may be defiberized by either chemical or mechanical methods. However, some species and defiberizing processes are better economic or functional matches than others.
An important difference between chemical and mechanical pulp is that mechanical pulp may be passed directly from the defiberizing stage to the paper machine. Chemical pulp on the other hand must be mechanically defiberized, washed and screened, at a minimum, after chemical digestion. Usually, chemical pulp is also mechanically refined after screening and prior to the paper machine. Additionally, the average fiber length of mechanical pulp is, as a rule, shorter than that of chemical pulp. However, fiber length is also highly dependent upon the wood species from which the fiber originates. Softwood fiber is generally about three times longer than hardwood fiber.
The ultimate properties of a particular paper are determined in large part by the species of raw material used and the manner in which the paper machine and web forming process treat these raw materials. Important operative factors in the mechanism of forming the paper web are the headbox and screen.
The particular fiber material or stock from which the paper is manufactured is, by nature, generally highly nonhomogeneous with respect to both the length and the thickness of the fibers. The longest fibers are of an order of 2 to 3 mm, while the shortest fibers are about 1/10 of this length. Only a few paper grades are produced by using a single fiber type alone. In most cases, at least two kinds of fiber are used for paper.
In conventional practice, a multiply board such as a three-ply board for packaging stock will contain as the middle or interior ply predominately softwood fibers with at least one of the outer plies containing predominately hardwood fibers. Generally speaking, hardwood fibers provide better smoothness as compared to softwood fibers, but are more expensive. On the other hand, softwood fibers confer higher strength and stiffness than hardwood fibers at a lower cost but at the expense of surface texture and smoothness unless the softwood fiber web is augmented by expensive fillers and other additives.
Also, most paper mills, for logistical and cost reasons and in order to be able to produce a commercially competitive product, must rely upon wood sources within the geographic area of the mill. The diversity of the local pulp sources vis-a-vis the natural ratio of softwood to hardwood therefore imposes a limitation on the mix of pulp available to the mill for making multi-ply board. In mills operating in regions containing predominately softwood pulp sources, hardwood pulp must often be transported to the mill from outside the region with a resultant economic penalty.
It is therefore an object of the present invention to provide a method for making multi-ply paperboard and, particularly, a three or more ply paperboard.
Another object of the invention is to reduce the total quantity of fiber per unit of web area (basis weight) in a multi-ply paperboard without a reduction in the web stiffness or caliper.
Also an object of the present invention is a balanced, three-ply paperboard of superior stiffness and surface texture.
An additional object of the present invention is a balanced, three-ply paperboard of superior stiffness and surface texture which can be produced economically using existing papermill equipment.
Still another object of the present invention is to enable production of multi-ply paperboard exhibiting improved properties with a reduction in total sheet weight at the same sheet stiffness.